Aqueous urethane polymers made their debut nearly 20 years ago. For their superior softness, mechanical quality and adhesiveness among various physical properties, such urethane polymers have been used widely in the field of coatings, binders, adhesives and the like. To cope with a recent movement of restricting the emission of volatile of organic compound (VOC) in the air from the viewpoint of environmental protection, for the development and application of aqueous coatings has been demanded strongly.
In order to produce urethane polymers suitable for use in aqueous polyurethane coatings, many attempts have been done to hydrophilize urethane polymers on the ground that hydrophilic groups therein render the high solubility or dispersibility in water. One notable example is an anionic-type urethane polymer which has basic salt of carboxyl groups attached to the prepolymer chain and which enjoys credit since it offers relatively good resistance to water. This type of urethane polymer is obtained by reacting a polyisocyanate, a polyol and dimethylol propionic acid in the mixture of a polar organic solvent having a high boiling point and an organic solvent having a low boiling point so as to form an urethane prepolymer having carboxyl groups and NCO groups at its terminal end, followed by neutralization of the resultant carboxyl groups with a basic material and by subsequent dispersion of the prepolymer in water (see Japanese Patent Publication Nos. 61-5485, 3-48955 and 4-488). After chain extending of the prepolymer with a polyamine compound, the solvents are then removed.
In the prior art methods noted above, the polar solvent having a high boiling point such as dimethylformamide (boiling point: 153.degree. C.), N-methylpyrrolidone (boiling point: 204.degree. C.) or the like should necessarily be used in combination with the organic solvent having a low boiling point in order to dissolve the dimethylol propionic acid uniformly. This latter compound is less soluble in the organic solvent having a low boiling point such as acetone, methyl ethyl ketone or the like. The use of the polar organic solvent having a high boiling point, however, poses the problem that removal or liberation of the solvent from the urethane prepolymer is difficult at a reaction stage after hydrophilization of the urethane prepolymer. It means that the polar solvent having a high boiling point is objectionably left unremoved in the final aqueous urethane polymer. A aqueous coating agent derived from the urethane polymer so formed results in a coat of poor drying. Another serious drawback is that the polar solvent having a high boiling point left in the coat acts like a plasticizer, eventually causing physical deterioration of the coat, or impairing coating environment due to some malodor inherent to that solvent. Moreover, water resistance of the coat is not always satisfactory.
To gain improved solubility of dimethylol propionic acid in the organic solvent having a low boiling point, it has been proposed as disclosed in Japanese Patent Laid-Open No. 6-313024 to use a hydrophilizing material for modifying the OH group of dimethylol propionic acid. The modified dimethylol propionic acid is more readily-soluble in the solvent. The hydrophilizing materials are, for example, a lactone monomer, for example such as .epsilon.-caprolactone, and an alkylene oxide such as ethylene oxide. The resulting urethane polymer, however, has still much to be improved with respect to its resistance of water and heat.
A reaction product of a urethane prepolymer and a chain-lengthening agent is also known, as disclosed in Japanese Patent Laid-Open No. 5-39340. The resulting urethane prepolymer can provide a coat with good resistance of hydrolysis as well as good appearance at low temperatures. Such prepolymer used here is composed of an organic diisocyanate, an amorphous polycarbonate diol and dimethylol propionic acid provided in its molecules with one hydrophilic group and two or more groups capable of reacting with the diisocyanate. The prepolymer, however, has still much to be improved with respect to its folwability and a control of the reaction for producing the prepolymer.